This invention relates to a regulating plate designed for improved contact with a flexible magnetic disk, and to a thin film magnetic head, more specifically to a thin film magnetic head having an improved protective layer stacked on a magnetic layer.
Recently, there has been developed an electronic camera system, which is now drawing attention, using an image pickup device such as a solid-state imaging device or an image pickup tube combined with a recording device using a flexible magnetic disk as a low-cost recording medium having a relatively large storage capacity to take the image of a subject as a pure elecctronic still picture, which is recorded on a rotating flexible magnetic disk, and the image is reproduced by a separate television or printer.
The flexible magnetic disk for such a system is used in the form of a flexible magnetic disk pack. Specifically, as shown in FIG. 5, a flexible magnetic disk 3, on which still picture information or the like is magnetically recorded, is rotatably contained in a flexible magnetic disk pack 2 which is installed in a fixed flexible magnetic disk unit 1. The flexible magnetic disk 3 is a magnetic recording medium with a diameter of about 50 mm, on which concentric tracks are formed from the inner periphery to the outer periphery. To rotate the flexible magnetic disk 3 at a high speed, a center core attached to the center of the flexible magnetic disk 3 is engaged with a spindle 6 of a motor 5. Further, there is provided a magnetic head 7 for magnetic recording on the flexible magnetic disk 7, which is movable in the radial direction. To achieve reliable recording on the flexible magnetic disk 3 which is rotating at a high speed, the magnetic head 7 must be in stable sliding contact with the flexible magnetic disk 3. However, since the flexible magnetic disk 3 has a flexibility, a stable contact is not achieved by simply pressing the magnetic head 7 against the flexible magnetic disk 3. Therefore, a regulating plate 8 is provided at the opposite side of the magnetic head 7 with respect to the flexible magnetic disk 3, to provide a stable contact of the flexible magnetic disk 3 with the magnetic head 7 by the function of an air flow between the regulating plate 8 and the flexible magnetic disk 3.
As a regulating plate for this purpose, for example, there has been developed one which is shown in FIG. 6. A regulating plate 9 is provided with protruding portions 10a and 10b at the upstream side and the downstream side with respect to the magnetic head 7, and with a hollowed portion 10c along the scanning direction of the magnetic head 7. Each of the protruding portions 10a and 10b has an inclined surface which becomes higher toward the downstream side in the rotational direction of the flexible magnetic disk 3 and becomes closer to the flexible magnetic disk 3. With this configuration, when the flexible magnetic disk 3 rotates, a positive pressure is generated between the protruding portions 10a and 10b and the flexible magnetic disk 3, and the flexible magnetic disk 3 is urged towards the magnetic head 7. In addition, at the outer peripheral side of the regulating plate 9 is provided a protruding portion 11 perpendicular to the radial direction. The protruding portion 11 is to hold the outer periphery of the flexible magnetic disk 3 which is subject to a large deflection, thereby achieving a uniform head touch over the whole tracks.
However, with such a positive pressure generated between the incline surfaces of the regulating plate 9 and the flexible magnetic disk 3, the regulating plate 9 is not always non-contacting with the flexible magnetic disk 3. It can rather be considered that the protruding portions 10a and 10b of the regulating plate 9 are pressing the flexible magnetic disk 3. Since, in particular, a positive pressure is not generated between the inclined surfaces of the regulating plate and the flexible magnetic disk at the start and stop of the spindle motor, the protruding portions 10a and 10b of the regulating plate 9 may become in contact with the flexible magnetic disk. Therefore, at the start and stop of rotation of the flexible magnetic disk, the flexible magnetic disk comes into contact with the regulating plate and may be scratched. In our actual experiments, an abrasion was observed on the protruding portions 10a and 10b of the regulating plate. In general, if the shape of the regulating plate changes due to abrasion, it will become difficult to maintain stable contact between the magnetic head 7 and the flexible magnetic disk 3, which will result in decreased output in recording and reproduction and deteriorated characteristics. We made investigation into changes in self-recording/reproduction output versus the number of start/stop cycles of the spindle motor, and obtained the result as shown in FIG. 4. In the figure, ENV (MIN/MAX) represents the ratio of valley to peak of the envelope curve of reproduced waveform, the greater is the value of the ratio, the better is the condition of contact between the magnetic head 7 and the flexible magnetic disk 3. Contrary to our expectation, it is shown that, although to a slight extent, contact condition between the magnetic head 7 and the flexible magnetic disk 3 improves with increasing number of start/stop cycles. This means that abrasion of the protruding portions 10 and 11 of the regulating plate 9 does not always have adverse effects but, to the contrary, provides an improved contact and hence improved results.
If it is the case, when the regulating plate 9 is previously formed to an abraded shape, a good contact between the magnetic head 7 and the flexible magnetic disk 3 should be obtained from the first. With an eye on the above fact, the present invention has thus been accomplished.
In the meantime, a thin film magnetic head comprises a substrate made of an abrasion-resistant material such as ferrite or sapphire, having on it a plurality of magnetic layers formed from sendust or an amorphous material, a coil conductor comprising an electroconductive metal, and an insulative layer, which are successively formed and etched into a predetermined pattern, and finally a protective layer formed to protect the magnetic layers from abrasion by a recording medium running on it.
It has been known for long, if the protective layer is sufficiently hard compared with the magnetic layer in the above-described configuration, abrasion due to running of the recording medium reaches the magnetic layer faster, which results in a biased abrasion in the magnetic layer, causing a spacing loss.
On the other hand, if the protective layer is too soft, overall abrasion proceeds faster, resulting in a shortened head life. Therefore, it is preferable that the protective layer is nearly as hard as, or slightly softer than the magnetic layer.
For example, for a magnetic layer with a Vickers hardness of Hv=600 to 650 kg/mm.sup.2, the protective layer is preferably set to a hardness of Hv=550 to 700 kg/mm.sup.2.
The protective layer, in view of the slidability of the recording medium and resistance to biased abrasion, is necessary to have a thickness of over 20 to 40 .mu.m. However, with such a thickness, the protective layer can generally undergo peeling or cracking due to an accumulated internal stress. Therefore, it is necessary to reduce internal stress to as small as possible. An effective means to solve the problems is to bring thermal expansion coefficients of materials of the head close to one another. However, in the past, it has been difficult to bring the thermal expansion coefficients of a metallic magnetic material and the protective layer close to each other.
Japanese Patent Publication Laid-open No. 62-16218/1987 discloses a mixture of MgO and SiO.sub.2 as a protective layer that can effectively suppress occurrence of biased abrasion. It also describes that an adequate hardness (Hv=450 to 850) and the desired thermal expansion coefficient can be obtained by setting the compositional ratio of MgO and SiO.sub.2 to 10 to 70% of SiO.sub.2.
However, the inventor of the present invention has conducted various experiments and found that the above MgO-SiO.sub.2 system has large flexibilities in terms of hardness and stress, but is inferior in water resistance, and has major defects for practical use. Specifically, for a preferable Vickers hardness range (over 550 kg/mm.sup.2), the MgO content is over 70 mole %, but such a material is inferior in water resistance, resulting in degradation and rusting at high temperatuare and humidity, which is not suitable for practical use. On the other hand, when the MgO content is below 70%, the water resistance is improved to a passable level, but the material is too soft, that is, the Vickers hardness is in an inadequate range, which is also not suitable for practical use.
Thus, it is a first object of the present invention to provide a regulating plate which has previously been formed to an abraded shape so that a good contact with the flexible magnetic disk is obtained from the initial stage. A second object of the present invention is to provide a thin film magnetic head with a protective layer which does not undergo biased abrasion and is superior in water resistance.